MATLAB GUI and Simulink

I have created a MATLAB GUI which works fine.

I also have a separate Simulink model that works fine as well from a script using the command:

options=simset('solver', 'ode8', 'fixedstep', timestep); sim('examplefile', t, options);

However, when i paste this code into my GUI, Simulink gives me an error:

Derivative input 1 of 'examplefile/Mass-Spring-Damper ODE/Integrator1' at time 0.0 is Inf or NaN. Stopping simulation. There may be a singularity in the solution.....

Any suggestions or examples of using the 'sim' command in a GUI.